A co-anchoring strategy for the synthesis of polar bimodal polyethylene

Since polar groups can poison the metal centers in catalysts, the incorporation of polar comonomers usually comes at the expense of catalytic activity and polymer molecular weight. In this contribution, we demonstrate polar bimodal polyethylene as a potential solution to this trade-off. The more-polar/more-branched low-molecular-weight fraction provides polarity and processability, while the less-polar/less-branched high-molecular-weight fraction provides mechanical and melt properties. To achieve high miscibility between these two fractions, three synthetic routes are investigated: mixtures of homogeneous catalysts, separately supported heterogeneous catalysts, and a co-anchoring strategy (CAS) to heterogenize different homogeneous catalysts on one solid support. The CAS route is the only viable strategy for the synthesis of polar bimodal polyethylene with good molecular level entanglement and minimal phase separation. This produces polyolefin materials with excellent mechanical properties, surface/dyeing properties, gas barrier properties, as well as extrudability and 3D-printability.

reported that is not consistent with this hypothesis. I would expect that either a Ni(0) complex (LHNiCOD) or Ni(II) complex (LNiCODH) would form, where CODH is bound to Ni as an alkyl or allylic subsituent. I have drawn some of these in the attached file. The NMR data are listed but not assigned, and the spectrum is not shown in the SI. Likewise the elemental analysis (both calculated and experimentally observed) was listed for the LNi fragment (C22H22NiO2P) rather than LNi(COD) (C30H35NiO2P). It is also unclear why Ni(COD)2 was used as a precursor rather than (py)2NiMe2, which was used for complexes 1 and 2, or other Ni(II) precursors such as Ni(allyl)Br dimer, py2Ni(CH2TMS)2, (PR3)2NiPhCl, etc. Although the syntheses of the catalysts is not really the focus of the paper, the lack of characterization for catalyst 3, or even a discussion of previous studies are concerning given that catalyst 3 is the one making the functionalized polymer. For this reason, I cannot recommend this manuscript for publication. I will be happy to re-review it once these questions are addressed. Zou and coworkers report a strategy for the synthesis of blends of two polymers: 1) a low MW fraction with high levels of polarity, and 2) a higher MW fraction with fewer polar groups. It is proposed that such a mixture will have the ideal properties of good mechanical properties, along with beneficial properties (dyeing, gas barrier, extrusion printing, etc). To achieve miscibility between these two components, three synthetic routes were explored: mixtures of homogeneous catalysts, separately supported heterogeneous catalysts, and a co-anchoring strategy. It was claimed that the co-anchoring strategy worked better than the other two strategies.
First, I feel that this paper addresses an important topic, and that the science here is excellent. However, I believe that the paper is fairly applied, and will likely be of interest to a select group of scientists working in the area of functional polyolefins, rather than a broad scientific audience. For these reasons, it is my opinion that this work would be better suited to a more specialized journal focusing on polymer synthesis. I would consider this for Nature Communications if the work were less empirical. For example, it is unclear to me how different the levels of functionality can be while still achieving miscibility. I would assume that at some gap in functionality, that the materials would phase separate. If the authors could make an array of PE materials with varying levels of functional group incorporation using single catalysts under controlled conditions, then map out the phase space for miscibility or phase separation, I would view this to significantly improve the scientific component of the paper. As it stands, I feel it would be better suited for a more specialized polymer journal.
Reviewer #1 (Remarks to the Author): In this paper mixtures of catalysts are co-anchored onto a MgO (or other) support using strong electrostatic adsorption previously developed by this group. I do not find this to be a weakness because the article focuses almost exclusively on the properties of the polymers as opposed to methodological development of catalyst properties. The behavior observed by the authors is not entirely unique, similar bimodal polymerization behavior was observed recently in a single site heterogeneous Pd catalyst catalyst that also incorporates polar monomers (ref 19), though that study's primary focus was catalyst development and detailed properties of the polymers were not discussed extensively. I do feel this study could be of interest to readers of Nature Communications, provided the following comments are addressed by the authors.

Gauss formula
Fig. S1. Peak fitting curves of molecular weight of polar bimodal copolymers in Table 1.
I am somewhat confused by Figure 1 h-1/h-2. These look like solution processed polymers, but this is not clearly stated. Also, the phase separation argument is not particularly strong here. I am surprised that the SEM in both examples are so smooth, it almost looks like nothing is there, which is surprising and not totally expected. Measuring these samples at high magnification is needed to support the arguments. Answer: Thanks a lot for your comments. According to the reviewer's comments, we have remeasured these samples at high magnification SEM (×5k, ×10k and ×20k) in the manuscript ( Fig. 1 h-1/h-2) and supporting information (Fig. S50). We have included the updated results and corresponding comments in the revised manuscript. These polymers were melt-pressed at about 150 °C, 5 MPa for 5 min to obtain the test specimen, and freeze it with liquid nitrogen and then wetting-off to obtain the fracture surface for SEM observation. As shown in Fig. S50A and B, obviously, the SEM image of polar bimodal polyethylene PPE-Ni2-MgO/Ni3-MgO indicated the presence of more phase separation than PPE-Ni2/Ni3-MgO.   Somewhat related, the 3D printing data is compelling, and I wonder if SEM data for products in Fig. 2H and 2I would be a more compelling proof-of-concept experiment showing how phase segregation does (or does not) occur with the PPE materials. Answer: Thanks a lot for your comments. According to the reviewer's comments, we have measured SEM of these samples in the supporting information (Fig. S55). We have revised the description in the manuscript, "The good surface properties of these polar bimodal polyethylene samples also enabled good compatibility with other types of polymers, making them more versatile for tuning material properties. For example, as shown in Fig. S55, after blending non-polar HDPE with polylactic acid (PLA), the SEM image showed obvious "sea-island" phase separation. However, the polar bimodal polyethylene PPE-Ni1/Ni3-APP with polar groups showed excellent polar compatibility, therefore, it was much easier to 3D print blends of PLA with polar bimodal polyethylene versus commercial HDPE (Fig. 4h vs 4i)." Reporting images of extrudate of PPE with mixtures of Ni2/MgO and Ni3/MgO should be given. Also, the images are not particularly clear, it was not obvious to me what I should be looking at in these images. Answer: Thanks a lot for your comments. According to the reviewer's comments, we have added the images extrudate of PPE with mixtures of Ni2/MgO and Ni3/MgO (PPE-Ni2-MgO/Ni3-MgO) in the manuscript, and got clearer pictures as follows.  We have revised the description in the manuscript, "Clearly, the extruded polar bimodal polyethylene sample (e2 and e3) was much smoother and more uniform than the sample with non-polar polyethylene (e1 , Table S2, entry 5, PE-Ni2-MgO). The introduction of polar functional groups and the presence of low-molecular-weight copolymers may have both contributed to its good extrusion performance. The extrusion performance of polar bimodal polyethylene prepared by co-anchoring strategy (e3 , Table 1, Entry 12, PPE-Ni2/Ni3-MgO) was better than that from separately mixed supported heterogeneous catalyst (e2 , Table 1, Entry 22, PPE-Ni2-MgO/Ni3-MgO), which further indicated that the two components of polar bimodal polyethylene prepared by co-anchoring strategy had better blending properties." A stylistic point, but one the authors should consider revising: many of the properties discussed on page 5 (dyeing, O2 permiability, and 3D printing) are written like a progress report. In addition, more description of how the gas barrier properties are measured would be helpful to the general audience. Answer: Thanks a lot for your comments. According to the reviewer's comments, we have revised the relevant description in the manuscript.
"Dyeing Properties. The dyeing properties of polyolefins are related to their surface properties. 40,41 The polar bimodal polyethylene and dye powder (2,2'-[(3,3'-Dichloro[1,1'biphenyl]-4,4'-diyl)bis(azo)]bis[N-(2-methylphenyl)-3-oxobutyramide]) were mixed. The blends were melt-pressed at 150 °C to obtain test specimens (Fig. 4c). Subsequently, the specimens were washed with hot acetone for 72 h and dried in a vacuum oven to a constant weight. In the UV-vis absorption spectra, the absorbance peak at 465 nm for polar bimodal polyethylene only decreased slightly after washing (Table 1, Entry 9). In contrast, the unimodal polar polyethylene with the same comonomer content (Table 1, Entry 7) showed a much more dramatic decrease. This is consistent with the surface property studies using WCAs and indicates the superior surface properties of the polar bimodal polyethylene prepared using the co-anchoring strategy, which further indicates that the surface of the film prepared by melting processing may contain more polar functional groups.
Gas Barrier Properties. The gas barrier properties of polymer films are very important for applications such as food packaging. After melting and pressing polar bimodal polyethylene and unimodal polyethylene prepared by different catalyst systems into thin films, we use GAS PERMEABILITY TESTER to measure the barrier of the film to oxygen. The results showed that the oxygen barrier properties of polar bimodal polyethylene prepared by co-anchored catalysts were even better than those of the unimodal polymer sample (Fig.4d; 3.22 vs 1.41,  3.84 vs 1.62). It is possible that the low-molecular-weight fraction, especially its good miscibility and entanglement with the high-molecular-weight fraction, made the film denser and improved its oxygen barrier properties. Furthermore, the bimodal copolymer prepared by separately mixing supported heterogeneous catalyst Ni2-MgO/Ni3-MgO showed poor oxygen barrier performance (5.02). This may be due to great phase compatibility of the polar bimodal polyethylene prepared by co-anchored catalyst (as reflected by SEM, Fig. 1h), leading to high molecular chain entanglement and the formation of dense film.
Extruding Properties. Bimodal polyethylene can improve processability. 7 The extrusion properties of polar bimodal polyethylene samples were studied with a single-screw extruder at 200 o C (Fig. 4e). Clearly, the extruded polar bimodal polyethylene sample (e2 and e3) was much smoother and more uniform than the sample with non-polar polyethylene (e1 , Table S2, entry 5, PE-Ni2-MgO). The introduction of polar functional groups and the presence of lowmolecular-weight copolymers may have both contributed to its good extrusion performance. The extrusion performance of polar bimodal polyethylene prepared by co-anchoring strategy (e3 , Table 1, Entry 12, PPE-Ni2/Ni3-MgO) was better than that from separately mixing supported heterogeneous catalyst (e2 , Table 1, Entry 22, PPE-Ni2-MgO/Ni3-MgO), which further indicated that the two components of polar bimodal polyethylene prepared by coanchoring strategy had better blending properties. The introduction of polar functional groups and the presence of low-molecular-weight copolymers may have both contributed to its good extrusion performance. 3D Printing. The emergence of three-dimensional (3D) printing has added a new dimension to polymer processing and holds huge prospects for manufacturing complex multi-functional material systems in a single processing step. 42,43 However, 3D printing high-density polyethylene (HDPE) has been problematic owing to its massive shrinkage, accompanied by its poor adhesion to common build plates. 44 Thus, it is difficult to 3D print commercial HDPE (Fig. 4f). The polar bimodal polyethylene material enabled by the co-anchoring strategy showed both improved extrusion properties and good surface properties. These properties made 3D printing viable for these polar polyethylene materials (Fig. 4g). The deliberately-chosen APP support also rendered the material flame retardant (Table 1, entry 19). The good surface properties of these polar bimodal polyethylene samples also enabled good compatibility with other types of polymers, making them more versatile for tuning material properties. For example, as shown in Fig. S55, after blending non-polar HDPE with polylactic acid (PLA), the SEM image showed obvious "sea-island" phase separation. However, the polar bimodal polyethylene PPE-Ni1/Ni3-APP with polar groups showed excellent polar compatibility, therefore, it was much easier to 3D print blends of PLA with polar bimodal polyethylene versus commercial HDPE (Fig. 4h vs 4i)." We modified the description of oxygen barrier performance test in the Methods of manuscript, "Gas barrier experiment. Oxygen permeability was determined by GAS PERMEABILITY TESTER (Basic 201, Jinan Languang Electromechanical Technology Co., Ltd) according to GB/T 1038-2000 at 25°C. The polymer was melted and pressed at about 150 °C, 5MPa for 10 minutes to obtain a film with the thickness of about 0.5 mm, and cut it into a circle with the diameter of 10 mm for gas barrier experiment. Install the prepared film into the vacuum chamber of the gas permeability tester, inject oxygen, measure the permeability of the film after a certain time, and prepare at least three samples of each polymer for testing." Several minor typos in the need to be corrected (D6H6, O2 barrea, etc) Answer: Thanks a lot for your comments. According to the reviewer's comments, we have corrected the typos in the manuscript and supporting information.

Reviewer #2 (Remarks to the Author):
The manuscript "A Co-Anchoring Strategy for the Synthesis of Polar Bimodal Polyethylene" describes the co-anchoring of Ni olefin polymerization catalysts to MgO for the purposes of making blends of high MW HDPE (made by catalyst 1 or 2) with lower Mw functionalized PE (made by catalyst 3). The performance of the supported catalysts is better than the homogeneous analogs, and co-anchored mixtures is better than mixtures of the individual heterogeneous catalysts. The tensile properties, rheology, melting behavior, wetting studies, oxygen permeability, and 3D printing results were reported and support the hypothesis that the polar functionality enhances the material properties of the PE blends. I have some questions about catalyst 3, however. The authors report that a complex with the formula (L)Ni(COD) is formed where L is the mono-deprotonated ligand; however, not enough information is provided. As written, the complex is formally Ni(I) and would be paramagnetic, but NMR data is reported that is not consistent with this hypothesis. I would expect that either a Ni(0) complex (LHNiCOD) or Ni(II) complex (LNiCODH) would form, where CODH is bound to Ni as an alkyl or allylic subsituent. I have drawn some of these in the attached file. The NMR data are listed but not assigned, and the spectrum is not shown in the SI. Likewise the elemental analysis (both calculated and experimentally observed) was listed for the LNi fragment (C22H22NiO2P) rather than LNi(COD) (C30H35NiO2P). It is also unclear why Ni(COD)2 was used as a precursor rather than (py)2NiMe2, which was used for complexes 1 and 2, or other Ni(II) precursors such as Ni(allyl)Br dimer, py2Ni(CH2TMS)2, (PR3)2NiPhCl, etc. Although the syntheses of the catalysts is not really the focus of the paper, the lack of characterization for catalyst 3, or even a discussion of previous studies are concerning given that catalyst 3 is the one making the functionalized polymer. For this reason, I cannot recommend this manuscript for publication. I will be happy to re-review it once these questions are addressed.
Answer: Thanks a lot for your comments. According to the reviewer's comments, we have obtained high quality characterization Ni3 catalyst and included the details in the revised manuscript and supporting information. Our initial attempt to react ligand L3 with (Py)2NiMe2 failed to give any isolable product. It may be due to the side reaction of this nickel precursor with the para-hydroxy group of ligand L3 with small steric hindrance. So we employed an alternative synthetic strategy for Ni3 according to the reference (J. Am. Chem. Soc. 2017, 139, 3611-3614). We have added corresponding characterization in the support information.
We have redrawn the structure of catalyst Ni3 and carefully characterized it as follows.

Reviewer #3 (Remarks to the Author):
Zou and coworkers report a strategy for the synthesis of blends of two polymers: 1) a low MW fraction with high levels of polarity, and 2) a higher MW fraction with fewer polar groups. It is proposed that such a mixture will have the ideal properties of good mechanical properties, along with beneficial properties (dyeing, gas barrier, extrusion printing, etc). To achieve miscibility between these two components, three synthetic routes were explored: mixtures of homogeneous catalysts, separately supported heterogeneous catalysts, and a co-anchoring strategy. It was claimed that the co-anchoring strategy worked better than the other two strategies.
First, I feel that this paper addresses an important topic, and that the science here is excellent. However, I believe that the paper is fairly applied, and will likely be of interest to a select group of scientists working in the area of functional polyolefins, rather than a broad scientific audience. For these reasons, it is my opinion that this work would be better suited to a more specialized journal focusing on polymer synthesis. I would consider this for Nature Communications if the work were less empirical. For example, it is unclear to me how different the levels of functionality can be while still achieving miscibility. I would assume that at some gap in functionality, that the materials would phase separate. If the authors could make an array of PE materials with varying levels of functional group incorporation using single catalysts under controlled conditions, then map out the phase space for miscibility or phase separation, I would view this to significantly improve the scientific component of the paper. As it stands, I feel it would be better suited for a more specialized polymer journal.
Answer: Thanks a lot for your comments. According to the reviewer's comments, we have supplemented the following experiments and discussions, including the study of the effects of different polar functional groups on the phase composition, mechanical properties, rheological properties, etc., in order to improve the scientific component of the manuscript.

1.
We have prepared a series of bimodal polyethylene with different comonomer incorporation ratios using co-anchored catalyst and mixed heterogeneous catalyst, and compared the phase compatibility, mechanical properties and rheological properties of these polyethylene, as shown in Table S4, Fig. 2, Fig. 3 and Fig. S51. In addition to the supported system of MgO, the phase separation behavior of the supported system of ammonium polyphosphate (APP) was also studied in Fig. S52.  undecenoic acid. b Yields are the average of at least two runs. Activity is in units of 10 5 g/(mol cat. × h). c Incorporation ratios of comonomers were determined from 1 H NMR spectra. d Determined by differential scanning calorimetry (DSC, second heating) e Mn: 10 4 g mol -1 , Mn, Mw, and Mw/Mn were determined by gel permeation chromatography in 1,2,4-trichlorobenzene at 160 o C. f T = 120 o C, 8 atm.

Fig. 2
Comparison of SEM images of two polar bimodal polyethylene prepared by co-anchored catalyst and mixed heterogeneous catalyst after incorporation of polar monomer. The upper polymers were prepared by co-anchoring strategy, and the lower polymers were prepared by mixed heterogeneous catalyst.   We have added the following discussions to the manuscript, "We have prepared a series of bimodal polyethylene samples with different comonomer incorporation ratios using co-anchored catalyst and mixed heterogeneous catalyst (Table S4), and compared their phase compatibility, mechanical properties and rheological properties. At low comonomer incorporation ratio (<0.5%), SEM images showed uniform homogeneity for both cases (Fig. 2). However, the samples prepared by mixed heterogeneous catalyst Ni2-MgO/Ni3-MgO showed obvious phase separation at incorporation ratios of above 0.9% (Fig.  2). In direct contrast, the samples prepared using co-anchored catalyst Ni2-Ni3/MgO maintained great compatibility even at high comonomer incorporation (1.7%).
Similar with the SEM results, the mechanical properties of the samples prepared by coanchoring strategy were only slightly decreased with increasing comonomer incorporation (0-1.7%) (Fig. 3, Fig. S51). However, the samples prepared by mixed heterogeneous catalyst showed extremely poor mechanical properties at comonomer incorporation ratio of above 0.5%, due to the obvious phase separation of the two components. In particular, the toughness of the material decreases sharply and almost disappears after the introduction of polar monomer. Clearly, the mechanical properties of bimodal polymer samples prepared by the two catalyst systems are quite different due to the differences of their microscopic phase separation behaviors.
The comparison of rheological properties of these samples also showed that the complex viscosity of bimodal polyethylene prepared by co-anchoring strategy were much higher than those prepared by mixed heterogeneous catalyst before the melting temperature, indicating that the two components of bimodal polyethylene prepared by co-anchoring strategy are more entangled. In addition, similar results were observed for other types of supported heterogeneous catalysts (APP) (Fig. S52)."

2.
According to the reviewer's comments, in order to improve the scientific content of the manuscript, we measured the SEM of these 3D printing samples in the supporting information to further study and verify the compatibility of polar bimodal polyethylene prepared by anchoring strategy with polar polymer (such as PLA) and the 3D printing performance of their blends. Fig. 4H and 4I in the manuscript. (a) and (b), SEM for HDPE: PLA 7:3 commercial (prepared by blending HDPE and polylactic acid in a ratio of 7 to 3). (c) and (d), SEM for PPE-Ni2/Ni3-APP: PLA 7:3 (prepared by PPE-Ni2/Ni3-APP and polylactic acid in a ratio of 7 to 3).

Fig. S55 SEM data for products in
We have revised the description in the manuscript, "The good surface properties of these polar bimodal polyethylene samples also enabled good compatibility with other types of polymers, making them more versatile for tuning material properties. For example, as shown in Fig. S55, after blending non-polar HDPE with polylactic acid (PLA), the SEM image showed obvious "sea-island" phase separation. However, the polar bimodal polyethylene PPE-Ni1/Ni3-APP with polar groups showed excellent polar compatibility, therefore, it was much easier to 3D print blends of PLA with polar bimodal polyethylene versus commercial HDPE (Fig. 4h vs 4i)."

3.
According to the reviewer's comments, in order to improve the scientific composition of the manuscript, we further studied the extrusion performance of polar bimodal polyethylene, and added the image extrusion of the mixture of PPE and Ni2/MgO and Ni3/MgO (PPE-Ni2-MgO/Ni3-MgO) to the manuscript, which was compared with the polar polyethylene prepared by anchoring strategy.  We have revised the description in the manuscript, "Clearly, the extruded polar bimodal polyethylene sample (e2 and e3) was much smoother and more uniform than the sample with non-polar polyethylene (e1 , Table S2, entry 5, PE-Ni2-MgO). The introduction of polar functional groups and the presence of low-molecular-weight copolymers may have both contributed to its good extrusion performance. The extrusion performance of polar bimodal polyethylene prepared by co-anchoring strategy (e3 , Table 1, Entry 12, PPE-Ni2/Ni3-MgO) was better than that of prepared by separately mixing supported heterogeneous catalyst (e2 , Table 1, Entry 22, PPE-Ni2-MgO/Ni3-MgO), which further indicated that the two components of polar bimodal polyethylene prepared by co-anchoring strategy had better blending properties." As an added note, I believe Ref 34 is incorrect. Answer: Thanks a lot for your comments. According to the reviewer's comments, we have corrected the Ref 34 in the manuscript.